Preparation method and application of ultra-microporous COF membrane with adjustable pore channel structure

Abstract

The invention belongs to the technical field of covalent organic framework membrane materials, discloses a preparation method of an aperture-adjustable ultra-microporous membrane constructed on the basis of ethynyl functionalized COF, and explores application of the aperture-adjustable ultra-microporous membrane in ion separation. The method comprises the following steps: firstly, constructing a COF membrane with an alkynyl structure by adopting a liquid-liquid interfacial polymerization mode, and then realizing precise cutting and functional modification of a COF pore channel structure through a copper-catalyzed azide-alkynyl cycloaddition reaction, so as to obtain the ultra-microporous COF membrane with adjustable pore diameter. By selecting azidation cutting agents with different structures, flexible regulation and control on the pore diameter structure of the membrane can be realized, and the separation selectivity on lithium and magnesium ions is remarkably improved. According to the membrane structure, the diffusion rate of magnesium ions is effectively reduced and the flux and selectivity of lithium ions are improved while the original frame crystallinity is maintained. The prepared cutting type COF film has good crystallinity, a uniform film surface structure and controllable aperture adjusting capability. The method is easy and convenient to operate, mild in condition and wide in application range, and the obtained membrane material has high stability and high selectivity and has good application prospects in efficient lithium extraction and separation of salt lake water resources with the high magnesium-lithium ratio.

Description

Technical Field

[0001] The invention relates to a preparation method and application of an ultra-microporous covalent organic framework separation membrane with an adjustable pore structure, belonging to the technical field of covalent organic framework membranes. Background Art

[0002] Li in salt lake brine + With Mg 2+ Coexistence and Mg 2+ The content is often Li + The traditional evaporation precipitation and solvent extraction methods are not only energy-intensive, but also have poor selectivity and serious secondary pollution. In recent years, membrane separation strategies based on differences in ion hydration diameters have attracted much attention due to their low energy consumption and high flux: Mg 2+ The hydrated diameter of Li is about 0.86 nm, while Li + The hydrated diameter of Li⁺ is about 0.76 nm, and the dehydration energy of Li⁺ (113.5 kcal / mol) is much lower than that of Mg²⁺ (437.4 kcal / mol), which makes Li + It is easier to dehydrate and pass through sub-nanopores in pressure or concentration driven separations. When the effective pore size of the membrane is reduced to 8.0 Å, Mg 2+ Highly efficient retention of Li + The high flux of Li + / Mg 2+ However, existing molecular sieves, metal-organic frameworks (MOFs), and other microporous material membranes struggle to achieve both high mechanical strength and defect-free fabrication when achieving highly uniform sub-nanometer pore structures, limiting their practical application.

[0003] As an emerging type of porous separation material, covalent organic framework (COF) membranes have shown broad application prospects in the fields of gas separation, liquid separation, and ion selective separation due to their designable organic building units and highly ordered pore structure. However, the common shortcomings in the synthesis of traditional COF membranes, such as large pore size and uneven pore size distribution, have seriously restricted the synergistic improvement of their flux and separation selectivity. In order to achieve a balance between high flux and high selectivity of COF membranes, it is urgent to develop a method that can precisely control the pore size while maintaining the orderliness of the COF framework. Pore Space Partition (PSP) was first applied to MOF materials. By constructing "gradient" or "segmented" functional areas in the pores, it can achieve precise screening of molecules or ions. Introducing the PSP concept into the field of post-modification of COF membranes requires overcoming technical difficulties such as the complex COF pore structure, limited space, and difficult to control reaction selectivity. Summary of the Invention

[0004] This paper addresses the technical bottleneck of existing COF membrane post-modification, which makes it difficult to achieve microscopic "cutting" and functional distribution. It proposes a method for constructing ultra-microporous COF membranes with pore cutting based on the copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC). The core of this method is: A COF monomer containing reactive alkyne groups is polymerized via interfacial polymerization or in situ growth to form a porous membrane on a free-standing or composite substrate. The resulting COF membrane exhibits a continuously tunable primary pore structure, with alkyne sites within the pores available for subsequent "click reactions." Under mild conditions, a Cu catalytic system consisting of a metal salt and a reducing agent is introduced into the COF membrane, followed by the addition of an azide reagent that efficiently reacts with alkyne groups. Through the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction, the original pore size is selectively "cut" within the COF membrane pores, and small molecules or functional clusters are simultaneously introduced, achieving precise pore size reduction and targeted distribution of functional groups. After the reaction, residual catalyst and unreacted reagents are removed through simple washing and drying, resulting in a structurally stable ultramicroporous functionalized COF membrane. The reaction time, temperature, and azide feed ratio can be adjusted to optimize the degree of pore cutting and functionalization distribution, further enhancing the membrane's permeability, selectivity, and long-term stability. The prepared ultramicroporous COF membrane was applied to ion selective separation, and its separation performance was evaluated using the separation factor. The results showed that the membrane obtained by the method of the present invention exhibited excellent selectivity in the ion separation process and the membrane structure was stable.

[0005] On the one hand, the present invention provides an ultra-microporous COF membrane. A COF membrane containing an alkyne group and an azide compound undergo an azide-alkyne cycloaddition reaction in the presence of a copper ion catalyst to achieve precise cutting and modification of the COF pore structure, thereby obtaining an ultra-microporous COF membrane with a pore size of 300-500 nm.

[0006] In some embodiments, the COF film containing alkyne groups is an acetylene COF film.

[0007] The azide compound includes either 4,4'-biphenyldibenzyl azide or 1,2-diazideethane. 4,4'-biphenyldibenzyl azide has a longer molecular structure and preferentially undergoes para-cleavage, significantly shrinking the COF membrane pore size. 1,2-diazideethane, on the other hand, has a shorter molecule and is more susceptible to ortho-cleavage, resulting in a smaller pore size adjustment range. By selecting azide compounds with different structural parameters, functionalized cleavage at different locations in the COF membrane pores is achieved, giving the system a degree of pore size adjustability and enabling on-demand pore construction.

[0008] The copper ion catalyst is a mixed solution of copper salt and sodium ascorbate.

[0009] In another aspect, the present invention provides a method for preparing an ultramicroporous COF membrane, comprising the following steps: Step 1: Modifying the surface of the PAN-based membrane with a NaOH solution to obtain a modified hydrophilic PAN-based membrane; Step 2: Fix the hydrophilic PAN-based membrane in an H-type diffusion cell, add the amino monomer solution and the aldehyde monomer solution to both sides of the hydrophilic PAN-based membrane, and allow them to stand for interfacial polymerization to obtain an acetylene COF membrane; Step 3: Add the copper salt solution to sodium ascorbate and mix well. Then add the azide cutting agent solution, mix well, and add it to the acetylene COF membrane in step 2. Perform the CuAAC reaction in a constant temperature shaker. After the reaction is completed, an ultra-microporous COF membrane with a pore structure is obtained.

[0010] In some embodiments, the preparation method of the hydrophilic PAN base membrane is to completely immerse the PAN base membrane in a NaOH solution to fully soak the base membrane; then transfer the soaked base membrane to a water bath for heating and hydrolysis reaction; after the reaction is completed, remove the base membrane, rinse it with deionized water until it is neutral, and dry it at room temperature to obtain a modified hydrophilic PAN base membrane (HPAN).

[0011] The concentration of the NaOH solution is 0.5-2 M. The actual volume used depends on the number of PAN-based membranes to be processed, and is approximately 10-20 mL per membrane, preferably sufficient to completely immerse the membrane material. The hydrolysis reaction temperature is controlled at 40-80°C, and the hydrolysis reaction time is controlled at 20-60 minutes.

[0012] The amino monomer is selected from any one of 2,5-diethynyl-p-phenylenediamine and 2,5-bis(prop-2-yn-1-yloxy)terephthalaldehyde; In some embodiments, the amino monomer solution is prepared by dissolving the amino monomer in a deionized water solution containing a certain proportion of glacial acetic acid, and then ultrasonically treating the solution to form a uniform and clear aqueous phase.

[0013] The molar concentration of the amino monomer is 7-100 mmol / L; and the volume fraction of glacial acetic acid is 0.02-0.5%.

[0014] The aldehyde monomer is selected from any one of 1,3,5-triformylphloroglucinol, tetrakis(4-formylphenylethylene), 1,3,5-phthalaldehyde, 1,3,5-tris(4-formylphenyl)benzene and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine.

[0015] In some embodiments, the aldehyde monomer solution is prepared by dissolving the aldehyde monomer in an organic solvent and then subjecting the solution to ultrasonic treatment to form a homogeneous organic phase.

[0016] The organic solvent is selected from acetonitrile, o-dichlorobenzene, mesitylene, N , N -dimethylacetamide and N , N -dimethylformamide, the amount used is the same as the volume of the aqueous phase.

[0017] The temperature of the interfacial polymerization reaction is 30-80° C., and the reaction time is 48-96 h to ensure sufficient growth of the COF film.

[0018] In some embodiments, after the reaction is completed, the membrane is removed and rinsed alternately with deionized water and anhydrous methanol to remove unreacted monomers and residual solvent; after rinsing, it is dried at room temperature to obtain a COF membrane with regular pores and acetylene functional groups.

[0019] In the step 3, the copper salt includes any one of CuSO4, CuCl2, CuI2 and CuBr2; The azide is selected from one of 4,4'-biphenyldibenzyl azide and 1,2-diazideethane; In some embodiments, the copper catalyst is prepared by the following method: copper catalyst solution A is prepared by dissolving a copper salt in a mixed solution of isopropyl alcohol and deionized water in a certain proportion, followed by adding sodium ascorbate in an amount equivalent to the molar amount of the copper salt, stirring and heating until fully dissolved and mixed, to form a copper catalyst active solution; Preparation of azide cutting agent solution B: dissolve the azide cutting agent in a mixed solution of isopropyl alcohol and deionized water in a certain proportion, and disperse by ultrasonication to form a uniform solution.

[0020] In the mixed solution of isopropyl alcohol and deionized water, the optimal volume ratio of isopropyl alcohol to water is 3:1; The concentration of the copper salt solution is 0.1 M, and the total volume of the mixed solvent is controlled at 20-40 mL; The molar ratio of sodium ascorbate to copper salt is 1:1; The heating and stirring temperature of the copper catalyst solution is 30 to 80°C.

[0021] In the step 3, the reaction temperature of the constant temperature shaker is 30-80° C., and the reaction time is controlled to be 4-12 hours.

[0022] In this paper, the COF membrane backbone refers to a two-dimensional covalent organic framework structure formed by the condensation reaction of aldehyde monomers and amino monomers, which has a highly ordered and predictable crystalline skeleton arrangement. This skeleton is constructed on the surface of a PAN-based membrane through liquid-liquid interfacial polymerization, forming a membrane material with a specific pore topology. For example, the TpPa-C≡C membrane composed of 1,3,5-triformylphloroglucinol (Tp) and 2,5-diethynyl-p-phenylenediamine (Pa-C≡C) exhibits a typical hexagonal ordered pore backbone.

[0023] The membrane skeleton structure was characterized by X-ray diffraction (XRD) analysis. The COF membranes all exhibited distinct diffraction peaks, indicating good crystallinity and a highly ordered skeleton structure capable of supporting stable pore pathways, providing a physical basis for subsequent screening behavior. Nitrogen adsorption-desorption analysis further verified the effect of cutting on pore size distribution. The initial COF membrane pore size was much smaller than that of the uncut membrane, demonstrating the successful construction of the pore-cut structure and significantly improving the screening selectivity of lithium and magnesium ions.

[0024] In this invention, COF membranes are named according to a unified convention: the abbreviations of the aldehyde monomer and amino monomer involved in the reaction are used to name the resulting COF membrane in the order of "aldehyde monomer abbreviation + amino monomer abbreviation." For example, the COF membrane obtained by the reaction of 1,3,5-triformylphloroglucinol (Tp) and 2,5-diethynyl-p-phenylenediamine (Pa-C≡C) is named TpPa-C≡C.

[0025] If the membrane is further treated with a cutting agent, a prefix is added before the name according to the site of the cutting method: when the cutting agent can act on the ortho position, it is marked as " o -", when the cutting agent acts on the counterpoint, it is marked as " p -". For example, TpPa-C≡C membrane modified with a para-cutting agent is named p -TpPa-C≡C, the adjacent cleavage is named o -TpPa-C≡C. This naming method is applicable to all COF membrane types involved in the present invention.

[0026] In another aspect of the present invention, the ultra-microporous COF membrane or the ultra-microporous COF membrane prepared by the method is used to separate Li + With Mg 2+ And selectively improve + application.

[0027] The present invention also provides a method for separating Li + With Mg 2+ And selectively improve +The membrane layer material includes the ultra-microporous COF membrane or the ultra-microporous COF membrane prepared by the method.

[0028] This invention belongs to the field of covalent organic framework (COF) membrane materials. It discloses a method for preparing an ultraporous membrane with adjustable pore size constructed from acetylene-functionalized COFs and explores its application in ion separation. This method first constructs a COF membrane with an acetylene structure using liquid-liquid interfacial polymerization. Subsequently, the COF pore structure is precisely cut and functionalized using a copper-catalyzed azide-acetylene cycloaddition reaction (CuAAC), resulting in an ultraporous COF membrane with adjustable pore size. By selecting a azidation cutting agent with different structures, the membrane pore size can be flexibly controlled, significantly improving the separation selectivity for lithium and magnesium ions. While maintaining the original framework crystallinity, this membrane structure effectively reduces the magnesium ion diffusion rate, thereby enhancing lithium ion flux and selectivity. The prepared cut-type COF membrane exhibits excellent crystallinity, a uniform membrane surface structure, and controllable pore size adjustment. This method is simple to operate, requires mild conditions, and has a wide range of applications. The resulting membrane material exhibits high stability and selectivity, and has promising application prospects in the efficient extraction and separation of lithium from high-Mg / Li salt lake water resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 These are the p-TpPa-C≡C membrane, o-TpPa-C≡C membrane, p-ETTBPa-C≡C membrane, and o-ETTBPa-C≡C membrane in the embodiments, where: ad correspond to the surface electron microscope images of the p-TpPa-C≡C membrane, o-TpPa-C≡C membrane, p-ETTBPa-C≡C membrane, and o-ETTBPa-C≡C membrane, respectively, and eh corresponds to the cross-sectional electron microscope images of the p-TpPa-C≡C membrane, o-TpPa-C≡C membrane, p-ETTBPa-C≡C membrane, and o-ETTBPa-C≡C membrane, respectively.

[0030] Figure 2 a, b and c are prepared as examples p -TpPa-C≡C membrane, o -TpPa-C≡C membrane, p -ETTBPa-C≡C membrane, o -Comparison of the performance of ETTBPa-C≡C membrane, TpPa-C≡C membrane, and ETTBPa-C≡C membrane, among which Figure 2 a is the Li of the film + Diffusion rate, Figure 2 b is Mg 2+ The diffusion rate, Figure 2 c is the Li of the film + / Mg 2+ Separation selectivity, where the separation selectivity is calculated by Li +Rate divided by Mg 2+ Rate obtained.

[0031] Figure 3 is the X-ray diffraction pattern of the film of Example 1, Example 2, Example 3, and Example 4, wherein: ad corresponds to TpPa-C≡C film, p -TpPa-C≡C membrane, ETTBPa-C≡C membrane, p -XRD pattern of ETTBPa-C≡C film.

[0032] Figure 4 is the Fourier transform infrared spectrum of the membrane of Example 1, Example 2, Example 3, and Example 4, wherein: ad corresponds to TpPa-C≡C membrane, p -TpPa-C≡C membrane, ETTBPa-C≡C membrane, p -Infrared image of ETTBPa-C≡C film.

[0033] Figure 5 The nitrogen adsorption and desorption pore size analysis of the membranes of Example 1, Example 2, Example 3, and Example 4, wherein ad corresponds to p -TpPa-C≡C membrane, TpPa-C≡C membrane, p -Nitrogen adsorption and desorption pore size analysis diagram of ETTBPa-C≡C membrane and ETTBPa-C≡C membrane. DETAILED DESCRIPTION

[0034] The specific implementation methods of the present invention are further described below with reference to the accompanying drawings and tables. The specific example implementation process described is only used to explain the present invention and is not intended to limit the present invention.

[0035] Example 1: (I) Modification of polyacrylonitrile (PAN) based membrane Weigh 4 g of NaOH and dissolve it in 50 mL of deionized water to prepare a 2.0 M NaOH solution. Soak a 3 cm diameter PAN membrane in this NaOH solution, ensuring complete immersion. Then, transfer the soaked PAN membrane to a water bath and heat at 60°C for 50 minutes to initiate the hydrolysis reaction. After the reaction is complete, remove the PAN membrane and rinse repeatedly with deionized water until the pH is neutral. Finally, air-dry the membrane at room temperature (25°C) for 12 hours to obtain the modified HPAN membrane for future use.

[0036] (2) Preparation of TpPa-C≡C membrane Weigh 0.3 mmol of the amino monomer 2,5-diethynyl-p-phenylenediamine, dissolve it in 20 mL of deionized water, add 20 µL of glacial acetic acid, and ultrasonically disperse it for 5 min to obtain a clear and homogeneous aqueous solution. Weigh 0.2 mmol of the aldehyde monomer 1,3,5-triformylphloroglucinol, dissolve it in 20 mL of o-dichlorobenzene, and ultrasonically disperse it for 5 min to obtain a uniform and transparent organic phase solution. The modified HPAN-based membrane was fixed in the middle of a homemade H-type diffusion cell. The prepared aqueous solution was then added to the non-woven side of the HPAN membrane, and the organic solution was added to the other side. The assembled diffusion cell was sealed and placed in a 60°C constant temperature oven for 72 h to complete the liquid-liquid interfacial polymerization process. After the reaction was completed, the membrane was removed and rinsed with deionized water and anhydrous methanol for more than three times to fully remove residual monomers and organic solvents; finally, the membrane was dried at room temperature for 12 hours to obtain a complete TpPa-C≡C membrane.

[0037] (III) Pore-cutting functionalization reaction of TpPa-C≡C membrane Prepare copper catalyst solution (solution A): Weigh CuSO4·5H2O (0.05 mmol) and add it to a mixture of 30 mL of isopropanol and deionized water (isopropanol: deionized water = 3:1, i.e., 22.5 mL of isopropanol and 7.5 mL of deionized water). Ultrasonicate for 5 min to fully dissolve and disperse it. Then, 0.05 mmol of ascorbic acid was added, and the mixture was heated and stirred in a 50 °C water bath for 20 min to make the copper catalyst solution completely homogeneous and clear; Prepare the azide cutting agent solution (Solution B): Weigh 0.15 mmol of 4,4'-biphenyldibenzyl azide and dissolve it in 9 mL of a mixed solvent of isopropanol and deionized water (isopropanol:deionized water = 3:1, i.e., 6.75 mL of isopropanol and 2.25 mL of deionized water). Ultrasonic dispersion is performed for 5 min to obtain a uniform solution of the azide cutting agent. Mix solution A and solution B at room temperature (25°C) and immediately immerse the TpPa-C≡C membrane prepared in step (2) in the mixed solution. Then, place the solution container containing the membrane in a constant temperature shaker at 200 rpm and 40°C for 8 h to complete the CuAAC reaction process. After the reaction, the membrane was taken out and rinsed with deionized water and anhydrous methanol for more than three times to completely remove the catalyst residues and unreacted azide on the membrane surface and in the pores. o C oven for 12 hours, and finally obtain a structure with precise channel cutting. p -TpPa-C≡C ultramicroporous covalent organic framework membrane.

[0038] (IV) Ion separation performance test of TpPa-C≡C membrane The prepared membrane sample was cut into circular pieces with a diameter of 2 cm and fixed in the middle of an H-type diffusion cell. The left and right sides of the diffusion cell were divided into two independent liquid pools, the raw material side and the permeate side. The raw material side liquid was configured as a mixed solution of 0.1 M LiCl and 0.1 M MgCl2 (volume 50 mL); an equal volume of pure deionized water was added downstream as the initial ion-free side. During the experiment, constant temperature stirring (about 300 rpm) was maintained upstream and downstream to reduce concentration polarization. 1 mL of downstream liquid sample was taken at the same time interval (e.g., every 10 minutes), and Li in the permeate was detected by inductively coupled plasma optical emission spectroscopy (ICP-OES) or ion chromatography. + and Mg 2+ The concentration change of ions is calculated based on the ion concentration difference within a fixed time, and Li is calculated by the following formula: + / Mg 2+ Selectivity coefficient (S Li+ / Mg2+ )

[0039] Among them, P(Li + ) represents Li + The diffusion rate of P(Mg 2+ ) represents the diffusion rate of Mg2+.

[0040] This strategy is also suitable for testing Li + / Na + 、Li + / K + To test the diffusion behavior of other ions in a multi-component system, the diffusion behavior can be controlled by changing the ion species in the mixed solution on the raw material side. If the separation performance under high lithium-magnesium ratio conditions needs to be evaluated, the preset Li + / Mg 2+ The raw material side solution is proportioned by molar ratio, while keeping the total ion concentration of the mixed solution constant at 0.1 M. For example, setting Li + / Mg 2+ If the molar ratio is 10:1, the raw material side solution can be configured as a mixed solution of 0.0909 M LiCl and 0.0091 M MgCl2, so that the total concentration of the two ions is controlled at 0.1 M, and a systematic study of the membrane separation performance under different concentration ratio conditions can be achieved.

[0041] Example 2 ( o -TpPa-C≡C membrane): The preparation method is the same as that of Example 1, except that in the pore cutting reaction of step 3, the cutting agent is replaced with 1,2-diazideethane (0.15 mmol), and other conditions remain unchanged. The membrane material finally obtained is recorded as o -TpPaC≡C.

[0042] Example 3 ( p -ETTBPa-C≡C membrane): Using 2,5-diethynyl-p-phenylenediamine (0.3 mmol) as the amino monomer and tetrakis(4-formylphenylethylene) (0.15 mmol) as the aldehyde monomer, liquid-liquid interfacial polymerization was carried out on the surface of the HPAN-based membrane according to the method in step 2 to prepare the ETTBPa-C≡C membrane.

[0043] The prepared ETTBPa-C≡C membrane was then treated according to step 3, using 4,4'-biphenyldibenzyl azide (0.3 mmol) as a cutting agent, CuSO4 (0.1 M, mixed solution volume 30 mL) as a copper catalyst, reaction temperature 40 °C, reaction time 8 h, and the final membrane material was recorded as p -ETTBPa-C≡C.

[0044] Example 4 ( o -ETTBPa-C≡C membrane): The preparation method is the same as that of Example 3, except that in the pore cutting reaction of step 3, the cutting agent is replaced with 1,2-diazideethane (0.6 mmol), and other conditions remain unchanged. The membrane material finally obtained is recorded as o -ETTBPa-C≡C.

[0045] Figure 1 For Example 1-4 p -TpPa-C≡C membrane, o -TpPa-C≡C membrane, p -ETTBPa-C≡C membrane, o -ETTBPa-C≡C membrane, where ad corresponds to p -TpPa-C≡C membrane, o -TpPa-C≡C membrane, p -ETTBPa-C≡C membrane, o -Surface electron microscopy images of ETTBPa-C≡C films, eh corresponds to p -TpPa-C≡C membrane, o -TpPa-C≡C membrane, p -ETTBPa-C≡C membrane, o -ETTBPa-C≡C membrane cross-section electron microscope image. Figure 1It can be observed that the surface of each membrane is dense and smooth, without obvious cracks or holes, and the overall surface morphology is uniform and coherent; the cross-sectional structure is compact and there is no stratification phenomenon, and the membrane thickness is moderate (about 300-500 nm), which ensures the mechanical stability of the membrane while achieving excellent ion screening performance.

[0046] Figure 3 is the X-ray diffraction pattern of the film of Example 1, Example 2, Example 3, and Example 4, wherein: ad corresponds to TpPa-C≡C film, p -TpPa-C≡C membrane, ETTBPa-C≡C membrane, p -ETTBPa-C≡C film XRD pattern. Figure 3 It can be seen that each film sample shows obvious diffraction characteristic peaks, indicating that the prepared COF film has good crystallinity and a highly ordered framework structure. p -TpPa-C≡C membrane, ETTBPa-C≡C membrane, p -ETTBPa-C≡C membranes all maintained the main diffraction peak positions, indicating that the orderliness of the COF skeleton was not destroyed during the pore cutting process and the membrane material structure was stable.

[0047] Figure 4 is the Fourier transform infrared spectrum of the membrane of Example 1, Example 2, Example 3, and Example 4, wherein: ad corresponds to TpPa-C≡C membrane, p -TpPa-C≡C membrane, ETTBPa-C≡C membrane, p -Infrared image of ETTBPa-C≡C film. Figure 4 It can be observed that all samples show obvious C≡C characteristic absorption peaks in the 2150–2164 cm-1 region, indicating that the alkynyl structure in the COF film is successfully retained; at the same time, characteristic absorption peaks of C=N functional groups appear at the 1653–1654 cm-1 position, further confirming the successful formation of the COF network skeleton.

[0048] Figure 5 The nitrogen adsorption and desorption pore size analysis of the membranes of Example 1, Example 2, Example 3, and Example 4, wherein: ad corresponds to p-TpPa-C≡C membrane, TpPa-C≡C membrane, p -Pore size analysis diagram of nitrogen adsorption and desorption of ETTBPa-C≡C membrane and ETTBPa-C≡C membrane. As can be seen from the figure, p -TpPa-C≡C membrane, TpPa-C≡C membrane, pThe average pore sizes of the α-ETTBPa-C≡C membrane and the ETTBPa-C≡C membrane were 0.65 nm, 1.37 nm, 0.58 nm, and 1.19 nm, respectively. These pore sizes were significantly smaller than those before cleavage. This change demonstrates that the introduction of the azide cleaving agent allows for precise control of the effective pore size of the COF membrane, further enhancing its performance for ion or small molecule separations.

[0049] Example 5 (unmodified TpPaC≡C membrane): Using 2,5-diethynyl-p-phenylenediamine (0.3 mmol) as the amino monomer and 1,3,5-triformylphloroglucinol (0.2 mmol) as the aldehyde monomer, liquid-liquid interfacial polymerization was carried out on the surface of the HPAN-based membrane according to the method of step 2 to prepare the TpPaC≡C membrane. In step 3, no cutting agent and copper catalyst were added, and only step 2 was performed. The membrane was rinsed and dried for later use.

[0050] Example 6 (unmodified ETTBPaC≡C membrane): Using 2,5-diethynyl-p-phenylenediamine (0.3 mmol) as the amino monomer and tetrakis(4-formylphenylethylene) (0.15 mmol) as the aldehyde monomer, liquid-liquid interfacial polymerization was carried out on the surface of the HPAN-based membrane according to the method of step 2 to prepare the ETTBPaC≡C membrane. In step 3, no cutting agent and copper catalyst were added, and only step 2 was performed. The membrane was rinsed and dried for later use.

[0051] Example 7 (pure HPAN membrane): After the PAN membrane was modified in step 1 (using 1 M NaOH solution, 60°C, reaction for 40 min), it was recorded as HPAN membrane. No COF monomer was added for interfacial polymerization reaction. Only the HPAN membrane was used, rinsed and dried for use as a blank comparison.

[0052] Example 8 (HPAN + 4,4'-biphenyldibenzyl azide treatment): The pure HPAN membrane (without COF) prepared in Example 7 was treated according to the method of step 3, and 4,4'-biphenyldibenzyl azide (0.3 mmol) was added as a cutting agent. Other experimental conditions were the same as those in Example 1. After the reaction was completed, it was rinsed and dried, and used as a control group.

[0053] Example 9 (HPAN + 1,2-diazideethane treatment): The pure HPAN membrane (without COF membrane) prepared in Example 7 was treated according to the method of step 3, and 1,2-diazideethane (0.3 mmol) was added as a cutting agent. Other experimental conditions were the same as those in Example 2. After the reaction was completed, the membrane was rinsed and dried, and used as a control group.

[0054] Example 10 The prepared p -TpPa-C≡C membrane, o -TpPa-C≡C membrane, p -ETTBPa-C≡C membrane, o -ETTBPa-C≡C membrane, TpPa-C≡C membrane, ETTBPa-C≡C membrane for Li + / Mg 2+ Separation selectivity experiments.

[0055] Figure 2 a, b and c are prepared as examples p -TpPa-C≡C membrane, o -TpPa-C≡C membrane, p -ETTBPa-C≡C membrane, o -Comparison of the performance of ETTBPa-C≡C membrane, TpPa-C≡C membrane, and ETTBPa-C≡C membrane, among which Figure 2 a is the Li of the film + Diffusion rate, Figure 2 b is Mg 2+ The diffusion rate, Figure 2 c is the Li of the film + / Mg 2+ Separation selectivity, where the separation selectivity is calculated by Li + Rate divided by Mg 2+ The rate is obtained. As can be seen from the figure, the membrane materials after cutting show significant improvement in the lithium-magnesium separation performance. p The lithium-magnesium selectivity of the -TpPa-C≡C membrane reached 133, which was significantly better than the uncut initial TpPa-C≡C membrane (selectivity was about 30), indicating that after cutting by 4,4'-biphenyldibenzyl azide, the pore size of the COF membrane was effectively shrunk, which significantly inhibited the diffusion and migration of magnesium ions and improved the membrane's screening efficiency for lithium ions.

[0056] o -TpPa-C≡C membrane also showed improved performance. Similarly, in the ETTBPa-C≡C system, the initial ETTBPa-C≡C membrane had a lithium-magnesium selectivity of 37. After cutting, p -The selectivity of ETTBPa-C≡C membrane is increased to 117, o -ETTBPa-C≡C membrane was increased to 75, further verifying the universality and effectiveness of the pore cutting strategy in different COF skeleton systems. In addition, the control experimental results showed that the modified HPAN membrane alone, or the membrane treated with a cutting agent without forming a COF skeleton, did not show effective ion separation ability. Specifically, the Li + / Mg2+ The selectivity was only 14.6, compared to 16 for the membrane with 4,4'-biphenyldibenzylazide and 18 for the membrane with 1,2-diazideethane, both far lower than the membrane material constructed with an ordered COF skeleton and regulated by pore cutting. This result further validates the synergistic and key role of the ordered structure of the COF membrane skeleton and pore cutting regulation in achieving efficient lithium-magnesium separation.

Claims

1. An ultra-microporous COF membrane, characterized in that: The COF membrane containing alkyne groups and azide compounds undergo an azide-alkyne cycloaddition reaction under the catalyst of copper ions to achieve precise cutting and modification of the COF pore structure, obtaining an ultra-microporous COF membrane with a pore size of 300-500nm.

2. The ultra-microporous COF membrane according to claim 1, characterized in that The COF film containing an alkyne group is an acetylene COF film.

3. The ultra-microporous COF membrane according to claim 1, characterized in that The azide compound includes one of 4,4'-biphenyldibenzyl azide and 1,2-diazideethane.

4. The ultra-microporous COF membrane according to claim 1, characterized in that The copper ion catalyst is a mixed solution of copper salt and sodium ascorbate.

5. A method for preparing an ultra-microporous COF membrane, characterized in that: The steps include: Step 1: Modifying the surface of the PAN-based membrane with a NaOH solution to obtain a modified hydrophilic PAN-based membrane; Step 2: Fix the hydrophilic PAN-based membrane in an H-type diffusion cell, add the amino monomer solution and the aldehyde monomer solution to both sides of the hydrophilic PAN-based membrane, and allow them to stand for interfacial polymerization to obtain an acetylene COF membrane; Step 3: Add the copper salt solution to sodium ascorbate and mix well. Then add the azide cutting agent solution, mix well, and add it to the acetylene COF membrane in step 2. Perform the CuAAC reaction in a constant temperature shaker. After the reaction is completed, an ultra-microporous COF membrane with a pore structure is obtained.

6. The method for preparing an ultra-microporous COF membrane according to claim 5, wherein: The amino monomer is selected from any one of 2,5-diethynyl-p-phenylenediamine and 2,5-bis(prop-2-yn-1-yloxy)terephthalaldehyde; The aldehyde monomer solution is selected from any one of 1,3,5-triformylphloroglucinol, tetrakis(4-formylphenylethylene), 1,3,5-phthalaldehyde, 1,3,5-tris(4-formylphenyl)benzene and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine; The temperature of the interfacial polymerization reaction is 30-80° C., and the reaction time is 48-96 h to ensure sufficient growth of the COF film.

7. The method for preparing an ultra-microporous COF membrane according to claim 5, wherein: The copper salt includes any one of CuSO4, CuCl2, CuI2 and CuBr2; The azide is selected from one of 4,4'-biphenyldibenzyl azide and 1,2-diazideethane; The reaction temperature of the constant temperature shaker is 30-80°C, and the reaction time is controlled to be 4-12h.

8. The ultramicroporous COF membrane prepared by the method according to any one of claims 5 to 7, characterized in that: The skeleton of the ultra-microporous COF membrane is constructed on the surface of the PAN-based membrane through liquid-liquid interfacial polymerization, forming a membrane material with a specific pore topology, which is a hexagonal ordered pore skeleton. The surface of the ultra-microporous COF membrane is dense and smooth, and the overall surface morphology is uniform and coherent; the cross-sectional structure is compact and has no stratification phenomenon, and the membrane thickness is 300-500 nm.

9. The ultra-microporous COF membrane according to any one of claims 1 to 4 or the ultra-microporous COF membrane prepared by the method according to any one of claims 5 to 7 is used to separate Li from salt lake brine. + With Mg 2+ And selectively improve + application.

10. A method for separating Li from salt lake brine + With Mg 2+ And selectively improve + The film material is characterized in that The invention comprises the ultramicroporous COF membrane according to any one of claims 1 to 4 or the ultramicroporous COF membrane prepared by the method according to any one of claims 5 to 7.

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